Paper Reviewed
Bunce, J.A. 2016. Responses of soybeans and wheat to elevated CO2 in free-air and open top chamber systems. Field Crops Research186: 78-85.

There is little doubt that nearly all plants respond positively to atmospheric CO2 enrichment, experiencing increases in photosynthesis, biomass and water use efficiency, to name but a few of the most commonly reported benefits. However, the magnitude of the CO2 response has remained a topic of debate given that different values have been derived from different experimental setups.

Primarily, there are two main systems designed for use in CO2 enrichment studies: open top chamber (OTC) and Free-Air-CO2-Enrichment (FACE). Built just as they sound, OTC systems are generally constructed out-of-doors in natural settings, where plants are grown inside relatively small diameter (less than 10 m) chambers constructed of polyethylene (plastic) walls that exceed the height of the plants. CO2-enriched air is usually pumped and released into the chambers via perforated tubes traversing the bottom of the ground, allowing for a fairly uniform concentration of CO2 to be controlled and maintained within the chamber.

FACE systems are also established in natural settings, but are constructed without walls. Their typical setup consists of several horizontal or vertical pipes that are placed around an experimental plot area that ranges from 1 to 30 m in diameter. CO2 enriched air is emitted from the pipes and the CO2 concentration is kept at the desired level via a system of strategically placed sensors that provide feedback to a computer that adjusts the flow of CO2 from the pipes, thus countering swings in CO2 that might arise from changes in wind speed and direction.

In general, OTC studies yield CO2-induced growth response increases that are significantly higher than those found in FACE studies. Considering FACE technology to be closer to the real world than the OTC setup, many climate alarmists -- including the IPCC -- have downplayed the benefits of atmospheric CO2 enrichment on plant growth. Not surprisingly, others have countered it is FACE that is the inferior technology (see, for example, a discussion on this topic here). The latest to weigh-in on this debate is Bunce (2015), who simultaneously conducted a FACE and OTC CO2 enrichment study on two cultivars of soybean and winter wheat over a period of two growing seasons.

Results of Bunce's analysis revealed that the approximate 200 ppm increase in CO2 enhanced the seed yield of soybean by 49 percent in the OTC setup, but by only 27 percent in the FACE setup. For wheat, elevated CO2 increased grain yield by 15-30 percent in the OTC setup for both cultivars in both years. In contrast, there was no increase in wheat grain yield for either cultivar in either year in the FACE study.

Pondering on why these differences occurred, Bunce explored and evaluated a range of possibilities, ultimately concluding that "the most likely explanation of the smaller crop yield increases at elevated CO2 in FACE than in OTC systems may be the greater fluctuation in [CO2] in elevated [CO2] treatments in FACE than in OTC." In other words, the FACE system design was incapable of maintaining a constant supply of elevated CO2, particularly during periods of high winds. In fact, according to Bunce, during the wheat growing season, "about one day in four had wind speeds of at least 10 ms-1, often for several hours per day, and the FACE system was not able to add CO2 fast enough to meet the target concentration at wind speeds above about 5-7 ms-1." In consequence of these deficiencies, the FACE plants experienced reductions in photosynthesis relative to plants growing in the OTCs that ultimately defined the difference in yields observed between the two technologies at the end of the growing seasons.

Clearly, therefore, the results of this study suggest it is the OTC experimental setup that is much more likely to provide the closest approximation of reality when it comes to estimating the future impact of atmospheric CO2 enrichment on plant growth and yield. And from the differences observed in Bunce's work, it would appear that FACE estimates are 20 to 30% too low.